WIP: [flang] Warn when F128 is unsupported

This generates `warning: REAL(KIND=16) is not an enabled type for this target`
if that type is used in a build not correctly configured to support this type.
Uses of `selected_real_kind(30)` return -1.

The added braces to the previous if statement are so that clang-format
does not try to indent the new comment. Even if the clang-format off
directive is moved to the start, that comment still gets indented so it
looks weird. I chose to disable clang-format for the preprocessor block
to allow the nested if statement to be indented for clarity.

Note: some lit tests will fail on systems not configured to support
f128. I'll update them once we are settled on what the right error
message is.

flang/include/flang/Tools/TargetSetup.h

@@ -11,6 +11,7 @@
 
 #include "flang/Evaluate/target.h"
 #include "llvm/Target/TargetMachine.h"
+#include <cfloat>
 
 namespace Fortran::tools {
 
@@ -21,9 +22,29 @@ namespace Fortran::tools {
 
   const llvm::Triple &targetTriple{targetMachine.getTargetTriple()};
   // FIXME: Handle real(3) ?
-  if (targetTriple.getArch() != llvm::Triple::ArchType::x86_64)
+  if (targetTriple.getArch() != llvm::Triple::ArchType::x86_64) {
     targetCharacteristics.DisableType(
         Fortran::common::TypeCategory::Real, /*kind=*/10);
+  }
+
+  // Figure out if we can support F128: see
+  // flang/runtime/Float128Math/math-entries.h
+  // clang-format off
+#ifdef FLANG_RUNTIME_F128_MATH_LIB
+  // we can use libquadmath wrappers
+  constexpr bool f128Support = true;
+#else
+  #if LDBL_MANT_DIG == 113
+  // we can use libm wrappers
+  constexpr bool f128Support = true;
+  #else
+  constexpr bool f128Support = false;
+  #endif // LDBL_MANT_DIG
+#endif // FLANG_RUNTIME_F128_MATH_LIB
+  // clang-format on
+
+  if constexpr (!f128Support)
+    targetCharacteristics.DisableType(Fortran::common::TypeCategory::Real, 16);
 
   targetCharacteristics.set_compilerOptionsString(compilerOptions)
       .set_compilerVersionString(compilerVersion);